1. Field of the Invention
The present invention relates to a thermal printer for use with a thermosensitive recording medium, which is provided with a thermal head and at least an optical fixing device. More particularly, the present invention relates to an optical fixing device of a color thermal printer, having a photosensor and a shutter for controlling the amount of light or electromagnetic rays applied for fixing the color thermosensitive recording medium.
2. Background Art
A color thermal printer for use with a color thermosensitive recording medium has been known. The color thermosensitive recording medium has at least three color thermosensitive layers which develop cyan, magenta and yellow when heated. Because the thermosensitive layers have different thermosensitivities for color, a full color image can be thermally recorded in a frame sequential fashion in order from the most sensitive to the least sensitive layer. After recording a color frame in the most sensitive layer, this layer is optically fixed by electromagnetic rays of a specific wavelength range, so as not to develop color any more. Then, a next color frame is recorded in the second sensitive layer, which is, thereafter, fixed by electromagnetic rays of another wavelength range. Finally, recording in the least sensitive layer is executed to complete a full color image.
A lamp for radiating the electromagnetic rays for fixing is mainly an elongated discharge tube, and disposed across a transport path of the thermosensitive recording medium. Because the intensity of light from the lamp changes with the temperature of the tube wall, as described, for example, in JP-Y-63-33321, it is desirable, for uniform optical fixing, to measure light from the lamp and maintain the light intensity of the lamp to be constant based on the measured light value, as is disclosed in JP-B-4-54590.
To avoid interference with the light path from the lamp to the recording medium, a photosensor may not be disposed between the lamp and the recording medium. It is possible to place the photosensor on a rear side of the lamp. However, for the sake of accurate measurement of illuminance on the recording medium, it is preferable to place the photosensor as closer to the recording medium as possible.
Moreover, luminance of the elongated discharge tube changes in the axial direction. That is, the luminance steeply decreases in proximity to the ends of the tube. Accordingly, accurate measurement is impossible with the photosensor being placed in the end portion of the lamp tube, even though the photosensor in that place does not hinder the light path from the lamp toward the recording medium.
Incidentally, there are plural types of printers: a circular type and a back-and-forth type.
In the circular type thermal printer, a sheet of the recording material is mounted around a periphery of a platen drum, to be transported circularly relative to a thermal head. A clamp member clamps an end of the recording sheet to position it on the platen drum. An advantage of the circular type is in that only small area along one end of the recording sheet is needed for positioning at the clamp member. An image can be recorded on the recording sheet only with small margin portions. However, as the platen drum must have a sufficiently large diameter enough to support a recording sheet around it, there is a limit in reducing the size of the circular type printer when it is required to record sheets of a B5 or A4 size. In addition, as the size of the platen drum becomes the larger, a drive motor for the platen drum must have the larger power. Therefore, the circular type is preferable for recording on a sheet of smaller size such as postcard size.
FIG. 10 schematically illustrates a conventional color thermal printer of a back-and-forth type. A pair 15 of transport rollers, consisting of a nip roller 15a and a drive roller 15b, first nip a front edge of a color thermosensitive recording sheet 10, and transport the recording sheet 10 in forward and reverse directions alternately: the forward direction from a supply tray 11 toward an exit passageway 16, and the reverse direction from the exit passageway 16 to a backward passageway 13.
The recording sheet 10 is squeezed between a platen roller 17 and a thermal head 18, while yellow, magenta and cyan colors are recorded to it. The platen roller 17 has a smaller diameter compared with the above-described platen drum that must support the entire length of the recording sheet 10 therearound.
Downstream from the transport roller pair 15 as viewed in the forward direction, there is disposed an optical fixing device 9, which includes a yellow fixing lamp 9a generating ultraviolet rays for yellow fixation, a magenta fixing lamp 9b generating ultraviolet rays for magenta fixation, and a reflector 9c for reflecting light from the fixing lamps 9a and 9b toward the recording sheet 10. While the recording sheet 10 is transported in the forward or the reverse direction, the color images are recorded to the coloring layers. Then the fixing device 9 is actuated while the recording sheet 10 is transported in the either of the forward and the reverse directions, to fix the coloring layers photochemically.
The back-and-forth type thermal printer is advantageous in reducing the size of the printer, as the diameter of the platen roller 17 is irrespective of the size of the recording sheet 10. However, the recording sheet 10 should be kept nipped between the transport rollers 15a and 15b during the thermal recording, so as to achieve an exact color registration with simple sheet transporting mechanism and control operation. Therefore, at least a length L1 of an edge portion of the recording sheet 10 must remain behind the transport roller pair 15, as shown in FIG. 10. As a result, an end portion L1+L2 of the recording sheet 10 extending from its edge 10a to a trailing end of the reflector 9c in the forward transport direction of the recording sheet 10 cannot be positioned in an illumination area A1 under the fixing device 9, so that this end portion L1+L2 is obliged to be a blank margin.
Especially in a back-and-forth type color thermal printer wherein the fixing lamps 9a and 9b arranged side by side along the transport path as illustrated in FIG. 10, the magenta fixing lamp 9b is located farther from the transport roller pair 15. As a result, even in a portion L4 which precedes to the end portion L1+L2 and is located in the illumination area A1 when the recording sheet 10 stops in the position shown in FIG. 10, a trailing half of the portion L4 remains unfixed with the magenta fixing lamp 9b. Moreover, exposure amount of the portion L4 to the fixing lamp 9a decreases from the leading end to the trailing end within the portion L4, as the exposure time decreases from the leading end to the trailing end. As a result, an uneven and insufficient fixing condition is provided in the portion L4.
Because such an insufficient fixing condition lowers the quality of the recorded image, it is necessary for a satisfactory quality to avoid recording in the portion L4 as well as in the portion L2+L1. Accordingly, the back-and-forth moving type printer needs to keep a quite large portion L3 (=L1+L2+L4) of the recording sheet 12 out of use for printing. That means the smaller area is available for printing on the recording sheet 10. Especially in such a back-and-forth type printer wherein more than one lamp or a U-shaped lamp is used for fixing one color to allow a higher transport speed of the recording sheet, the illumination area A1 is correspondingly elongated, so that the available area of the recording sheet 10 would be still more reduced.